A mask (may also be referred to as a photomask or reticle) is a device that physically stores a pattern. The pattern is transferred to a wafer by lithography. Wafer includes a semiconductor wafer, an LCD or OLED display, or a magnetic storage medium. Lithography includes ultra violet (UV), deep ultra violet (DUV), extreme ultraviolet (EUV) lithography, electron or X-ray projection lithography, and nano imprint lithography (NIL). In UV and DUV lithography, the mask is a glass plate that is transmissively imaged on to the wafer with an optical lithography projector. In DUV the mask may be a binary mask (a pattern etched in a chromium film on a glass plate), an attenuated phase-shift mask (a pattern etched in a molybdenum silicide film on a glass plate), an alternating phase-shift mask (a first pattern etched into a chromium film on a glass plate and a second pattern etched into the glass substrate), or a chrome-less phase-shift mask (a pattern etched into a glass plate with no opaque material). In EUV lithography, the mask is reflectively imaged on to a wafer. The glass plate or a plate made of other low-thermal-expansion-coefficient material is coated with multiple layers of molybdenum and silicon to achieve high reflectivity. An absorber film such as tantalum-boron-nitride is coated over the multi-layer and a pattern is etched into the absorber film. In NIL, in particular step and flash imprint lithography (SFIL), a pattern is etched into a glass plate that is also referred to as a “template.” In electron and X-ray projection lithography, the mask comprises an opaque stencil, which is a thin film such as nickel with a pattern etched through the film.
The patterning and mask technologies described above are given as background examples. They should not be construed to limit this invention to a particular patterning technology. Each wafer usually has many patterned layers, and a separate mask is used to pattern each layer. A mask set for all layers is used to manufacture many wafers. The layers of an advanced semiconductor device need to overlay with a very tight tolerance, on the order of few nanometers (nm). One of the contributors to wafer overlay is mask registration. Features defined on a mask may be slightly displaced from their ideal locations during fabrication. Such displacements are called registration error or registration for short. Registration of each mask manufactured is measured by a mask registration metrology system. If registration does not meet specifications, the mask is discarded and the mask writing equipment and process may be adjusted to ensure that pattern placement is accurate and repeatable. Traditionally, metrology targets are used to measure registration. Metrology targets are patterns that are specific to metrology and they are not part of the circuit that is formed on the semiconductor wafer. Metrology targets are inserted in the mask layout with a clear area surrounding each target.